The present invention is directed to a sleeve for increasing the retention value of a screw in a structural component, particularly in a railroad tie. The sleeve has an external thread with a thread revolution and a constant external diameter on the outer surface of the sleeve and an internal bore which at the trailing end of the sleeve has a thread revolution of an internal thread with a pitch corresponding to the thread revolution of the external thread.
In the past and even at the present time railroad track is laid on wooden ties. The connection of the panels carrying or supporting the rails with the wooden ties are made with known screws equipped with a thread similar to a wood screw. The retaining values of such screws diminish with the passage of time, because of weathering, the aging of the wood and continuous dynamic loads. As a result, there is a lessening of the positive locking connection between the receiving material in the form of the wooden tie and the thread of the screw, with the development of a weak point caused by the destruction of the thread in the bore of the wooden tie by the screw thread.
To again provide adequate retention values, a rehabilitation of the connection region is indispensable. The rehabilitation of the receiving material could be effected by replacing the wooden tie, however, for economic reasons a rehabilitation of the bore in the wooden tie has been considered. Various methods having different disadvantages have been known for such rehabilitation of the bore, where it has been attempted to provide a sound undamaged thread in the wooden tie for the screw threads.
Apart from chemical rehabilitation methods, which have disadvantages due to the application technology, the use of metallic helixes has been widely accepted. Such a device, disclosed in EP 0 188 490 effects a lining of the ineffective internal thread within the bore of the wooden tie for providing the possibility of threading-in the screw to form a new internal thread in the undamaged receiving material between the revolutions of the helix. While the use of such a helix has a negative effect on material costs when viewed purely economically, another particular disadvantage is the cumbersome setting method. The setting method is performed so that in a first work step the screw has to be withdrawn, in an additional work step the helix has to be threaded into the existing internal thread in the bore using a separate tool and, only after the setting tool is withdrawn from the hole, can the screw be threaded-in as a final work step by cutting a new internal thread. Accordingly, a separate work step is needed for inserting the helix and, in addition, requires the use of a special setting tool.
A plastics material sleeve with an external thread is disclosed in DE 1 784 010, however, it is not intended for such rehabilitation purposes and is also used in concrete ties. This plastics material sleeve acts as a type of dowel and assures an adequate positive lock due to a special shaping of its exterior contour in cooperation with the wooden tie.